Mitigation of sub-synchronous control interaction in DFIGs using a power oscillation damper

Performance evaluation of SSCI damping controller based on the elastic energy equivalent system

Strong sub-synchronous control interactions caused by open-loop modal resonance between the DFIG and series-compensated power system

Mitigation of Sub-Synchronous Control Interaction in Wind Power systems with GA-SA tuned Damping Controller

Interaction between fixed series compensation and doubly-fed induction generator (DFIG)-based wind farms can cause a new type of subsynchronous resonance (SSR) problem, which is also named as subsynchronous control interaction (SSCI). To explain its basic mechanism as well as to evaluate the risk, this paper proposes a small-signal impedance method, based on which, a practical series-compensated power system including DFIGs can be simplified into a second-order series RLC circuit around the operating point concerned and then the frequency and stability of SSR can be intuitively represented by the equivalent parameters of the circuit. The derivation of the circuit parameters is given and the asymmetry phenomenon of equivalent impedance matrix in dq0 reference frame is observed. Both Eigenvalue analysis and time-domain simulation have verified the effectiveness of the proposed method and its advantage in accuracy over the existing impedance-based method. The impacts on SSR due to wind speed, number of online DFIGs and their control parameters are also investigated.

Many wind turbine generators (WTGs) are installed in remote, rural areas, where the power grids are usually weak, characterized by unbalanced voltage conditions. If the voltage unbalance is not taken into account in the control system, it will cause poor power quality and poor operating performance of the WTG systems. This paper proposes a novel control scheme to improve the operating performance of a wind turbine equipped with a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. The rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG are controlled in a positive (dq)+ reference frame as well as in a negative (dq)- reference frame. Control of the RSC and GSC in the (dq)+ reference frame is the same as the regular control under balanced network conditions. The supplementary control of the RSC in the (dq)- reference frame minimizes the electromagnetic torque pulsations of the DFIG caused by the unbalanced network voltage; the supplementary control of the GSC in the (dq)- reference frame contributes to balance the total output currents of the DFIG. The proposed control scheme is implemented in PSCAD/EMTDC on a 3.6 MW DFIG wind turbine connected to a power network with unbalanced voltage. Results show that it improves the operating performance of the DFIG wind turbine system.

The adverse effects resulting from the intermittent nature of wind energy on power generation and stability is well documented in the literature. More specifically, the impact of power oscillations on doubly fed induction generator (DFIG)-based wind turbines can have drastic effects and even lead to large area blackouts, if not addressed properly. This paper deals with the design and implementation of a probabilistic coordinated approach for power systems with synchronous generators (SGs), DFIGs, and facts devices. The parameters of the power system stabilizers (PSSs) of the SG, power oscillation dampers (POD) of the DFIGs and STATCOM controllers were optimally coordinated using a modified Imperialist Competitive Algorithm (MICA) combined with a probabilistic eigenvalue approach. The proposed approach was implemented to a modified 39-bus New England power system. Its effectiveness in guaranteeing power system's dynamic stability was assessed using time domain analysis, eigenvalue mapping, and robustness analysis under various wind conditions. DFIG's realistic modes of operation, including normal, synchronous, and sub-synchronous modes were considered in the performance analysis. The obtained results were further compared to those of deterministic and uncoordinated approaches. A comparison of the MCA algorithm to other optimization approaches such as particle swarm optimization (PSO), genetic algorithm (GA), and imperialist competitive algorithm (ICA) is also carried out. The simulation results and robustness analysis confirmed the superiority of the proposed probabilistic approach over various other approaches.

Damping low frequency oscillations of power systems is one of the most important determinants of system stability. Due to the increasing growth of wind farms with high penetration levels in power systems as well as their participation and control, it seems important to increase the damping system oscillations. In this paper, doubly fed induction generators (DFIGs) were used for damping the power oscillations. For this purpose, a fuzzy controller as a power oscillation damper (FPOD) was applied to DFIG control loop as backup control and also input and output scales of fuzzy controller were optimized by particle swarm optimization (PSO) algorithm. The main reason was use this hybrid approach is to enhance disturbance rejection and reduction, through fuzzy systems. In order to clarify the DFIG capability in mitigating the inter-area oscillations, non-linear simulations were carried out by MATLAB/Simulink. Simulation study in a four-machine two-area power system show that the DFIG with a FPOD controller was superior to the conventional POD in terms of stabilizing effect as well as robustness against various power generating and loading conditions, unpredictable network structure, and random wind patterns.

This article presents the sharing of reactive power between two converters of a doubly fed induction generator (DFIG) based wind energy conversion system interacting with the grid. The rotor side converter (RSC) control of DFIG is designed for sharing of reactive power at below rated wind speeds, which essentially reduces the amount of rotor winding copper loss. However, at rated wind speed, the RSC control is designed to maintain the unity power factor at stator terminals and to extract rated power without exceeding its rating. Further, the reduction in rotor winding copper loss due to reactive power distribution is demonstrated with an example. Moreover, the grid side converter (GSC) control is designed to feed regulated power flow to the grid along with reactive power support to DFIG and to the load connected at point of common coupling. Moreover, the GSC control is designed to compensate load unbalance and load harmonics. The battery energy storage connected at dc link of back-to-back converters, is used for maintaining the regulated grid power flow regardless of wind speed variation. The system is modeled and its performance is simulated under change in grid reference active power, varying wind speed, sharing of reactive power, and unbalanced nonlinear load using SimPowerSystems toolbox of MATLAB. Finally, a prototype is developed to verify the system steady state and dynamic performance. Moreover, system voltages and currents are found sinusoidal and balanced, and their total harmonic distortions are as per the IEEE 519 standard.

Under the disruption of communication system, the wide area controller may fail to stabilize power oscillations. As a result, the power grids may be jeopardized due to the undamped oscillations. To improve the grid resiliency, a new design method of the two-level robust coordinated stabilizing control of power oscillation damper (POD) of wind turbine with doubly-fed induction generator (DFIG), and power system stabilizer (PSS) is presented in this paper. The two-level control of POD and PSS consists of centralized and local levels. As the main controllers, the centralized POD and PSS are applied to damp out power oscillations. When the communication failure occurs, the local POD and PSS act as the backup controllers to stabilize power oscillations instead of the centralized POD and PSS. The geometric measures of controllability and observability are adopted to determine the suitable input signals of POD and PSS. The structure of POD and PSS is represented by a practical 2nd-order lead/lag compensator. The control parameters of POD and PSS of each control level are separately optimized under various operating conditions so that the damping effect and robustness can be guaranteed. Simulation study is conducted to show the stabilizing effect of the proposed controller on the enhancement of grid resiliency against severe short circuits, line outages, various power flow levels, wind speeds, variable communication latency, and communication failure.

This paper proposes a Co-ordination control of doubly fed induction generator (DFIG) under generalized distorted grid voltage conditions. The system behavior of DFIG during supply imbalance and voltage distortion from grid side is investigated. To improve the fault ride-through operation capability of DFIG, the rotor side converter (RSC) and grid side converter (GSC) are appropriately regulated to eliminate the torque oscillations and ensure constant active power output from overall DFIG generation system. In this simulation model co-ordination control technique is proposed to achieve simultaneous regulation of the positive / negative-sequence currents of DFIG, discrete PID controller is implemented in the positive reference frame. The Matlab simulation results demonstrate the correctness of the developed technique and the usefulness of the proposed control strategy for DFIG based wind turbine system under such adverse grid conditions.

A quick post fault recovery of wind generator system could be achieved with the help of a series dynamic braking resistor (SDBR). This paper investigates the dynamics, size, optimal location of a series dynamic braking resistor in the doubly fed induction generator (DFIG) structure based on different switching strategy of the braking resistor. Three signals were investigated in switching the SDBR during transient conditions of the DFIG wind generator; the DC-link voltage, rotor current and grid voltage, respectively. Also, three locations were considered in connecting the SDBR in the DFIG system; the stator, rotor and grid side converter, respectively. Furthermore, the optimum size and best insertion time of the SDBR were investigated based on the most effective performance of the SDBR scheme considering its switching signal and position in the DFIG system. Simulations were run in power system computer-aided design and electromagnetic transient including DC (PSCAD/EMTDC). The results show that when the SDBR is connected to the DFIG, improved performance was achieved; however, the stator based SDBR topology in conjunction with a grid voltage actuating signal, fast insertion time and short duration of operation quickly recovered the DFIG wind generator variables during transient conditions.

Approximate dynamic programming based supplementary reactive power control for DFIG wind farm to enhance power system stability

The paper analyzes the capability of a wind power plant, based on doubly-fed induction generators (DFIGs) with an improved control system, to fulfill low voltage ride-through (LVRT) capability of the wind turbines providing the reactive power support. According to grid code requirements, tripping of wind generators under grid faults is not allowed. The sensitivity of DFIGs to grid faults requires the use of crowbar protection for not damaging the electronic equipment. In this paper, the model of a wind power plant based on DFIG using active crowbar protection is developed in Matlab/Simulink. Nevertheless, the results indicate that DFIGs equipped with crowbar protection can protect the electronic equipment, but the rotor side converter is blocked due to crowbar activation and the reactive power control is lost. Therefore, for improving the LVRT, a coordinated control system of the grid side converter, active crowbar protection, and rotor side converter is implemented and proposed, The grid side converter of DFIG is controlled similar to a STATCOM, considering also the control parameters of the crowbar protection. The simulation results show that the proposed coordinated control grid side converter'active crowbar-rotor side converter can supply the reactive power in order to answer the grid codes requirement for LVRT. Therefore, this control system can be useful to improve the LVRT capability of wind power plants based on DFIG.

This work proposes an enhanced Low-Voltage Ride Through (LVRT) nonlinear control of Doubly Fed Induction Generator (DFIG)-based wind turbines under symmetric three-phase voltage dips in stiff grid conditions. Most of the control schemes that use rotor currents control for LVRT act on the stator flux by injecting reactive power into the grid so to re-arise the stator voltage to accepted values. Indeed, when the grid at the DFIG coupling point is powerful enough, the stator flux is no more controllable. This investigation first gives a mathematical model for the wind generation system besides a Backstepping control scheme for grid-connected DFIG powers flow control. Then, effects of a three-phase symmetric voltage dip are analysed and the reasons of rotor current surges, due to the voltage dip, and factors influencing it are inferred. As the stator flux is imposed by the power grid, this work proposes a Backstepping control for both converters to keep DFIG connected to grid within accepted values during faults. The rotor side converter keeps DFIG currents at nominal values and limits transients. The grid side converter keeps the voltage at DC-link stable. Results of these theoretical findings are verified by MATLAB/SIMULINK simulation on a 300 kW wind generation system.

The ESS is considered as an effective tool for enhancing the flexibility and controllability of a wind farm, and the optimal control scheme of a wind farm with distributed ESSs is vital to the stable operation of wind power generation. In this paper, a coordinated active and reactive power control strategy based on model predictive control (MPC) is proposed for doubly fed induction generator (DFIG)-based wind farm (WF) with distributed energy storage systems (ESSs). The proposed control scheme coordinates the active and reactive power output among DFIG wind turbines (WTs), grid-side converters (GSCs), and distributed ESSs inside the WF, and the aim is to decrease fatigue loads of WTs, make the WT terminal voltage inside the extent practicable, and take the WF economic operation into consideration. Moreover, the best reactive power references of DFIG stator and GSC are produced independently based on their dynamics. At last, the control scheme generates optimal power references for all ESS to make the SOC of each ESS converge to their average state. With the distributed ESSs, the WF controller regulates the WTs inside WF more flexibly. A WF composed of 10 DFIG WTs was utilized to verify the control performance of the proposed coordinated active and reactive power control strategy.